LCD or liquid crystal displays have found widespread use in modern television and computer screens, wristwatches, calculators and the like. A large proportion of modern LCD displays are based on the twisted nematic liquid crystal due to Helfirch and Schadt (Swiss patent CH532261). The orientation of light entering the device is twisted by liquid crystals which are oriented in a spiral or corkscrew fashion. Due to polarizers at the entrance and exit of the device, only light that is thus twisted will exit the device. Upon application of an electric field normal to the liquid crystal plane ('up' or ‘down’ the stairs), the ability of the liquid crystal to twist light orientation is impaired, blocking light from passing through the device and causing the device to appear darker shades of grey for increasing fields, eventually reaching black for a high enough field. Many such individual devices can be fabricated in close proximity to form an LCD screen. Due to the directional nature of the twisted nematic liquid crystals, light incoming from directions off-normal will be less well modulated by the liquid crystal effect mentioned above. Therefore the viewing angle e.g. of laptop screens is reduced. Great effort has been expended to increase the viewing angle of such devices. However this effect can actually be used to advantage, by effectively blocking the light incident from certain angles and allowing the rest to pass. This use of the directional nature of light absorption by liquid crystals is novel and as will be shown below is of great utility in several applications. In brief, since the disclosed apparatus can block incoming light from a particular direction (such as that of the sun) while passing light from other directions, it is capable of reducing glare and increasing the dynamic range of a scene viewed by a camera, a driver, a pilot, a house occupant, etc. Since the system is electronically controlled, an open- or closed-loop feedback system can modify the direction of greatest light attenuation adaptively to track bright objects and keep them blocked.
U. S. patent application 20060209250 discloses a beam steering device using a liquid crystal with an array of back electrodes. Voltages are applied to the array to cause a desired phase distribution across the array, the distribution being selected to steer a beam incident upon the array into a desired direction. Reflective elements are disposed to reflect light incident in the spaces between the electrodes to reduce losses and to smooth the transitions in phase between adjacent electrodes. However the system is not adapted for the selective transmission or absorption of light based on angle of incidence, but rather to steer a beam incident from a known direction in a controlled fashion.
European patent EP0151703 discloses a directional filter for ambient light constructed from a thin base strip of indeterminate length and having an opaque surface. The strip is wound into a roll having a plurality of convolutions and sections are cut from the face of the roll. A pair of sections are disposed so that their convolutions are in an orthogonal configuration, and are sandwiched between a pair of glass plates having non-reflective outside surfaces. Channels are thereby provided, which impart directional characteristics to the ambient light. However the system is not adapted for the selective transmission or absorption of light based on angle of incidence, but rather to passively impart directional characteristics to the incoming light.
Similarly European patent EP0658780 discloses a directional filter, characterized by a plurality of lamellae which form mutually parallel beam wells, the interspaces being filled by transparent support bodies whose refractive index causes the incident light to be refracted towards the normal on the plane of incidence. Again the system is not adapted for the selective transmission or absorption of light based on angle of incidence, but rather to passively impart directional characteristics to the incoming light. Patents EP0122830 and U.S. Pat. No. 4,621,898 follow similar lines to EP0658780.
Hence, a system for a directional filter is still a long felt need.